Overview- Willamette River spring Chinook salmon represent an economically and culturally important species of conservation need. Tributary impoundment by dams lacking fish passage limits access to Willamette River basin spawning areas. This limitation is mitigated by trapping, hauling, and outplanting spring Chinook into upstream spawning habitats. However efficacy of outplanting efforts has been limited by high prespawn mortality (PSM) levels. Research to understand spring Chinook PSM has focused on quantifying: 1) migration duration and timing, 2) pathogen loads of fish throughout the migration period, and 3) the effect of holding captured spring Chinook in ambient and pathogen free hatchery water. Migration duration and timing of individually tagged fish at trapping facilities varied over the migration period presumably in response to varying hydro-thermal regimes. Additionally, fish arriving at trapping facilities later in the migration had higher pathogen loads; however loads varied between years and outplant basins. Preliminary results indicate that higher pathogen loads were associated with PSM in outplanted fish, but rearing under hatchery conditions during the summer months resulted in reduced PSM levels relative to in-river estimates.

As part of this project I will be devloping a model to synthesize the complex interactions of migration duration and timing with pathogen loads on spring Chinook PSM, and to provide a tool to evaluate alternative management strategies that may minimize PSM of outplant fish. The model also identified knowledge gaps and uncertainties.